The growth hormone axis: control and effects

Coated cysteamine, a potential feed additive for ruminants - An updated review

For sustainable development, better performance, and less gas pollution during rumen fermentation, there is a need to find a green and safe feed additive for ruminants. Cysteamine (CS) is a biological compound naturally produced in mammalian cells. It is widely used as a growth promoter in ruminants because of its ability to control hormone secretions. It mainly controls the circulating concentration of somatostatin and enhances growth hormone production, leading to improved growth performance. CS modulates the rumen fermentation process in a way beneficial for the animals and environment, leading to less methane production and nutrients loss. Another beneficial effect of using CS is that it improves the availability of nutrients to the animals and enhances their absorption. CS also works as an antioxidant and protects the cells from oxidative damage. In addition, CS has no adverse effects on bacterial and fungal alpha diversity in ruminants. Dietary supplementation of CS enhances the population of beneficial microorganisms. Still, no data is available on the use of CS on reproductive performance in ruminants, so there is a need to evaluate the effects of using CS in breeding animals for an extended period. In this review, the action mode of CS was updated according to recently published data to highlight the beneficial effects of using CS in ruminants.

Dietary Cysteamine Supplementation Remarkably Increased Feed Efficiency and Shifted Rumen Fermentation toward Glucogenic Propionate Production via Enrichment of Prevotella in Feedlot Lambs

Cysteamine (CS) is an essential nutritional regulator that improves the productive performance of animals by regulating somatotropic hormone secretion. To investigate the fattening potential and effects of CS on rumen microbial fermentation, 48 feedlot lambs were randomly assigned to four groups and fed diets supplemented with different CS concentrations (0, 20, 40, and 60 mg/kg BW). An increase in dietary CS concentrations linearly increased the average daily gain (ADG) and dry matter intake (p < 0.05) but decreased the feed-to-gain ratio (p < 0.01). For the serum hormone, increasing the dietary CS concentration linearly decreased somatostatin and leptin concentration (p < 0.01) but linearly increased the concentration of growth hormone and insulin-like growth factor 1 (p < 0.01). Regarding rumen fermentation, ruminal pH, ammonia-N, and butyrate content did not differ among the four treatments, although dietary CS supplementation linearly increased microbial protein and propionate and decreased the amount of acetate (p < 0.05). Furthermore, an increase in dietary CS concentrations quadratically decreased the estimated methane production and methane production per kg ADG (p < 0.05). High-throughput sequencing revealed that increased dietary CS concentrations quadratically increased Prevotella (p < 0.05), and Prevotella and norank_f__norank_o__Clostridia_UCG-014 were positively correlated with growth performance and rumen fermentation in a Spearman correlation analysis (r > 0.55, p < 0.05). Overall, a CS concentration higher than 20 mg/kg BW produced growth-promoting effects by inhibiting somatostatin concentrations and shifting the rumen toward glucogenic propionate fermentation by enriching Prevotella. In addition, Prevotella and norank_f__norank_o__Clostridia_UCG-014 were positively correlated with growth performance in lambs.

Effects of plasmid-mediated growth hormone-releasing hormone supplementation on LL-2 adenocarcinoma in mice

This study was designed to measure the effects of plasmid growth hormone-releasing hormone (GHRH) supplementation on LL-2 (Lewis lung adenocarcinoma) tumor-bearing immunocompetent mice. Male and female mice (n = 20/group/experiment) received 2.5 x 10(6) LL-2 cells in the left flank. One day later, we injected the mice intramuscularly with 20 micro g of a myogenic plasmid, pSP-hGHRH or pSP-betagal, as a control. Mean serum IGF-I was significantly higher in treated animals versus controls (P < 0.05). Male and female mice constitutively expressing GHRH exhibited a decline in tumor growth rate relative to controls (20% for males, P < 0.03, and 11% for females, P < 0.13). Histopathological analysis revealed that treated animals were less likely to develop lung metastases than controls (11%) and had no alternate-organ metastases. The number of metastases/lung was reduced by 57% in female mice with GHRH treatment (P < 0.006). When tumor size exceeded 8% of body weight, GHRH-treated mice showed normal urea, creatinine, and kidney volume, while controls displayed signs of renal insufficiency. This study provides evidence that with plasmid-mediated GHRH supplementation in tumor-bearing mice, tumor growth rate is not increased but is actually attenuated.

Maternal GHRH plasmid administration changes pituitary cell lineage and improves progeny growth of pigs

Previous studies from our laboratory have demonstrated that administration of a myogenic plasmid that encodes a protease-resistant growth hormone-releasing hormone (HV-GHRH) to pregnant rat dams augmented long-term growth in first-generation progeny. In the present study, gilts were injected intra-muscularly at day 85 of gestation with 0, 0.1, 0.5, 1, or 5 mg of the HV-GHRH-expressing plasmid and were then electroporated. Piglets were weighed and bled periodically from birth to 100 kg. Piglets from gilts treated with 1 and 5 mg of HV-GHRH plasmid were larger at birth and weaning compared with controls. These two groups reached 100 kg 9 days earlier than the other groups. GHRH levels were increased at birth in piglets from treated gilts. IGF-I levels were significantly increased in the 5-mg group beginning at 21 days of age compared with controls. Pituitaries from the 5-mg group contained a significantly increased number of somatotrophs and lactotrophs from birth to 100 kg. This study confirms that enhanced maternal GHRH production results in intergenerational growth augmentation and that the magnitude of the response is dose dependent. The similarity of the response across species suggests that the effect is likely exerted as a fundamental component of gestational and developmental physiology.

Nonhereditary enhancement of progeny growth

The im electroporated injection of a protease-resistant GH-releasing hormone cDNA into rat dams at 16 d gestation resulted in enhanced long-term growth of the F(1) offspring. The offspring were significantly heavier by 2 wk of age, and the difference was sustained to 10 wk of age. Consistent with their augmented growth, the plasma IGF-I concentration of the F(1) progeny was increased significantly. The pituitary gland of the offspring was significantly heavier and contained an increased number of somatotrophs and PRL-secreting cells, which is indicative of modification of cell lineage differentiation. These unique findings demonstrate that enhanced GH-releasing hormone expression in pregnant dams can result in intergenerational growth promotion by altering development of the pituitary gland in the offspring.

Enhanced animal growth via ligand-regulated GHRH myogenic-injectable vectors

Regulated animal growth occurred following a single electroporated injection of a mixture of two plasmids (10 microg of DNA), one expressing the GeneSwitch regulator protein, the other an inducible growth hormone releasing hormone (GHRH) gene, into the tibialis anterior muscles of adult SCID mice. Administration of the ligand mifepristone (MFP) up-regulated GHRH expression, as shown by elevations of IGF-I levels, and when MFP dosing was withdrawn, IGF-I returned to baseline levels. Five cycles of IGF-I induction were observed during a five-month period. Chronic MFP dosing for 25 days increased lean body mass, weight gain, and bone mineral density significantly compared with non-MFP treated controls. In summary, long-term drug-regulated GHRH expression was achieved following plasmid-based gene therapy, and chronic induction of GHRH expression in adult animals led to improvements in weight gain and body composition.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Relationship between the thymus and neurochemical changes in the hypothalamus-preoptic area and prefrontal cortex in female rats with delayed puberty

In female rats, aged 55-58 days with delayed puberty due to deficient growth and environmental stress, 5-hydroxyindoleacetic acid levels and serotonin turnover rate in the hypothalamus-preoptic area as well as body weight, body weight gain and relative weight of ovaries, uterus, adrenals and preputial glands were lower while serotonin and 5-hydroxyindoleacetic acid levels in the prefrontal cortex were higher when compared to normal rats with the latest onset of puberty aged 42-52 days. In rats with delayed puberty, multiple regression analysis revealed a significant negative dependence on dopamine turnover in the hypothalamus-preoptic area for body weight gain and, of all organs, for the relative weight of the thymus. A similar negative significant dependence on serotonin turnover rate in the prefrontal cortex was also found for the relative weight of thymus and spleen. The same analysis in the opposite direction revealed a significant negative dependence of 3,4-dihydroxyphenylacetic acid levels and dopamine turnover rate in the hypothalamus-preoptic area as well as serotonin turnover rate in the prefrontal cortex only on thymus weight. After separation of delayed pubertal rats into two groups, based on absolute ovarian weight, the rats in the low ovarian weight range and no signs of puberty exhibited: lower body weight gain, lower body weight, and lower relative weight only of thymus, ovaries and preputial glands in parallel with an increased dopamine turnover rate in the hypothalamus-preoptic area and serotonin turnover rate in the prefrontal cortex compared to the delayed pubertal rats in the high ovarian weight range and early signs of puberty. The results suggest that in rats with delayed puberty: (1) serotonergic activation in the hypothalamus-preoptic area is lower compared to normal puberty rats; (2) dopaminergic activation in the hypothalamus-preoptic area negatively affects body weight gain, thymus weight and initiation of puberty and (3) thymus weight is negatively implicated in dopaminergic activation in the hypothalamus-preoptic area and serotonergic activation in the prefrontal cortex and positively related to ovarian weight and early signs of puberty.

Effect of route and frequency of administration of apomorphine on growth hormone release in African catfish (Clarias gariepinus)

Apomorphine is known to stimulate growth hormone release in African catfish following an intraperitoneal (IP) injection. In the present study the effect of apomorphine (5 or 20 mg/kg body weight) on plasma GH levels was evaluated after gastro-intestinal or parenteral delivery. Apomorphine increased the plasma GH concentration regardless of the route of administration, indicating that apomorphine can be absorbed from the intestinal tract. The effect of repeated administration of apomorphine differed clearly between the tested doses. Although a single IP injection with 20 mg apomorphine/kg body weight resulted in a clear increase in plasma GH levels, a second injection given 12 hours later was ineffective. In contrast the last of 4 consecutive injections with 5 mg apomorphine/kg body weight given at intervals of 12 hours stimulated the plasma GH levels in a similar way to a single IP injection with the same dose.